Flotation systems are important unit operations in process engineering technology that were developed to separate particulate constituents from slurries. Flotation is a process for liquid and solid separation and also separation of insoluble liquids whereby gas is bubbled through a suspension of finely dispersed particles, and the hydrophobic particles are separated from the remaining slurry or mixture by attachment to, or entrainment by the gas bubbles. The gas bubble and particle aggregate, formed by adhesion or entrainment of hydrophobic particles to gas bubbles, is generally less dense than the slurry or mixture, thus causing the aggregate to rise to the surface of the flotation vessel. Flocculant dosing of the slurry or mixture also may be used to aid in the entrainment of hydrophobic particles. Separation of the hydrophobic particles is then accomplished by separating the upper layer of the slurry, which is in the form of a froth or foam, from the remaining slurry or mixture by hydrodynamic means or mechanical means or both,
The fundamental step in froth flotation involves gas bubble to particle contact for a sufficient time to allow the particle to rupture the gas-slurry film and thus establish either attachment to or entrainment by gas bubbles. The total time required for this process is the sum of contact time and induction time, where contact time is dependent on bubble and particle motion and on the hydrodynamics of the system, whereas induction time is affected by the surface chemistry properties of the bubbles and particles. Traditional methods of creating conditions for froth flotation include the use of simple aerated or agitated cells, or chambers as termed herein, including flotation cells, flotation columns, direct air flotation cells with or without flocculant dosing, attrition scrubbers, heavy media separators, or impeller cells, and non-conventional flotation devices like vessels described in U.S. Pat. Nos. 4,838,434, 4,279,743, 4,397,741, 4,399,027, 4,744,890, or the device disclosed by authors in U.S. Pat. No. 5,192,423. However, flotation separation has certain limitations that render flotation cells and flotation columns inefficient in many applications. Particularly, in the past it has been recognized that conventional flotation is not very effective for the recovery of fine particles (less than 10 microns in diameter). This can be a serious limitation, especially in the separation of fine minerals. An explanation for this poor recovery is that particle momentum in traditional flotation devices is so slow that particle penetration of the gas-slurry film is inhibited, thus resulting in poor rates of attachment to the bubbles. Furthermore, conventional flotation has never been relied on as a process to effect separation of hydrocarbons in slurry.
A further limitation of conventional flotation systems is that nominal particle retention times in the order of several minutes are required to achieve successful separation. However, it has been shown that particle to gas bubble attachment occurs frequently in the order of milliseconds, therefore indicating that the rate of separation is mostly limited by low bubble-to-particle collision probability or transport or both rather than by other factors. As such, these necessary long retention times severely limit plant capacity and require the construction of relatively large and expensive equipment. Traditional flotation technology uses counter current flows, and multiple stages for the gas to be introduced to slurry. While these traditional methods and associated apparatus do achieve particle to gas bubble attachment, they are inefficient, requiring long processing times and consequent large equipment volumes. The inefficiency associated with the traditional prior art approaches arises largely from the relatively low gas to slurry volume ratio provided by the equipment.
That fact stipulates that the key in achieving high efficiency is in generating a high gas to slurry volume ratio. When very small bubble size and narrow size distribution is achieved then a high gas to slurry volume ratio is generated. The smaller the bubble the bigger the number of gas bubbles that can be packed into the unit volume. That translates directly to higher probability of bubble to particle collision and subsequent attachment or entrainment. The process of generating bubbles is dynamic and equilibrium must be achieved between creation of new bubbles and bubbles coalescing into bigger ones.
Inventive discoveries related to the present invention include that optimum bubble size distribution can be only achieved if a porous tubular housing with mean pores size below 100 microns is used for the gas diffuser. It has also been discovered that optimum conditions exist for a given range of G forces and Reynolds number, which impart limitations on flow rates and the diameter of diffuser. High Reynolds numbers promote maintaining small bubble size and so prevent bubble coalescence by ripping apart all bubbles bigger than eddies in the flow. However, too high G force quickly moves bubbles to the centre due to the buoyancy of the bubbles. Once at the centre, the flow becomes coaxial with consequently drastically lower Reynolds number and bubbles coalescence that rapidly lowers interfacial contact surface area.
Flotation machines function to provide the hydrodynamic and mechanical conditions that effect actual separation. Apart from the obvious requirements of feed entry and tailings exit from cells and banks and for hydrodynamic or mechanical froth removal, to be effective, the cell, or chamber as termed herein, typically also provides:                1. Effective suspension and dispersion of small particles to prevent sedimentation and to permit their contacting gas bubbles;        2. Influx of gas, bubble formation, and bubble dispersion;        3. Conditions favoring particle to bubble contact and either attachment or entrainment;        4. A non-turbulent surface region for stable froth formation and removal; and        5. In some cases, sufficient mixing for further mineral reagent interaction.        